Copper(I)-Catalyzed Asymmetric Alkylation of Unsymmetrical Secondary Phosphines

Chiral bisphosphine Ph-BPE ligand: a rising star in asymmetric synthesis

Chiral 1,2-bis(2,5-diphenylphospholano)ethane (Ph-BPE) is a class of optimal organic bisphosphine ligands with C2-symmetry. Ph-BPE with its excellent catalytic performance in asymmetric synthesis has attracted much attention of chemists with increasing popularity and is growing into one of the most commonly used organophosphorus ligands, especially in asymmetric catalysis. Over two hundred examples have been reported since 2012. This review presents how Ph-BPE is utilized in asymmetric synthesis and how powerful it is as a chiral ligand or even a catalyst in a wide range of reactions including applications in the total synthesis of bioactive molecules.

Palladium-catalyzed regio- and stereo-selective phosphination of cyclic biarylsulfonium salts to access atropoisomeric phosphines

A palladium-catalyzed regio- and stereo-selective phosphination of cyclic biarylsulfonium salts (racemic) with HPAr3Ar4 for straightforward synthesis of atropoisomeric phosphines (P,S-ligands) bearing a stereogenic axis or both a stereogenic axis and a P-stereogenic center is reported. The high reactivity and regio- and stereo-selectivity originate from the torsional strain release and palladium catalysis, and the construction of a P-stereogenic center is enabled by an efficient dynamic kinetic resolution. The high performance of the nascent P,S-ligands has been demonstrated in palladium-catalyzed asymmetric allylic substitutions, indicating the great potential of the present methodology.

Second Generation Catalytic Enantioselective Nucleophilic Desymmetrization at Phosphorus (V): Improved Generality, Efficiency and Modularity

A broadly improved second generation catalytic two-phase strategy for the enantioselective synthesis of stereogenic at phosphorus (V) compounds is described. This protocol, consisting of a bifunctional iminophosphorane (BIMP) catalyzed nucleophilic desymmetrization of prochiral, bench stable P(V) precursors and subsequent enantiospecific substitution allows for divergent access to a wide range of C-, N-, O- and S- substituted P(V) containing compounds from a handful of enantioenriched intermediates. A new ureidopeptide BIMP catalyst/thiaziolidinone leaving group combination allowed for a far wider substrate scope and increased reaction efficiency and practicality over previously established protocols. The resulting enantioenriched intermediates could then be transformed into an even greater range of distinct classes of P(V) compounds by displacement of the remaining leaving group as well as allowing for even further diversification downstream. Density functional theory (DFT) calculations were performed to pinpoint the origin of enantioselectivity for the BIMP-catalyzed desymmetrization, to rationalize how a superior catalyst/leaving group combination leads to increased generality in our second-generation catalytic system, as well as shed light onto observed stereochemical retention and inversion pathways when performing late-stage enantiospecific SN2@P reactions with Grignard reagents.

Asymmetric synthesis of P-stereogenic phosphindane oxides via kinetic resolution and their biological activity

The importance of P-stereogenic heterocycles has been widely recognized with their extensive use as privileged chiral ligands and bioactive compounds. The catalytic asymmetric synthesis of P-stereogenic phosphindane derivatives, however, remains a challenging task. Herein, we report a catalytic kinetic resolution of phosphindole oxides via rhodium-catalyzed diastereo- and enantioselective conjugate addition to access enantiopure P-stereogenic phosphindane and phosphindole derivatives. This kinetic resolution method features high efficiency (s factor up to >1057), excellent stereoselectivities (all >20:1 dr, up to >99% ee), and a broad substrate scope. The obtained chiral phosphindane oxides exhibit promising therapeutic efficacy in autosomal dominant polycystic kidney disease (ADPKD), and compound 3az is found to significantly inhibit renal cyst growth both in vitro and in vivo, thus ushering in a promising scaffold for ADPKD drug discovery. This study will not only advance efforts towards the asymmetric synthesis of challenging P-stereogenic heterocycles, but also surely inspire further development of P-stereogenic entities for bioactive small-molecule discovery.

Pd(II)-Catalyzed enantioselective C-H olefination toward the synthesis of P-stereogenic phosphinamides

P-Stereogenic phosphorus compounds are important structural elements in chiral ligands or organocatalysts. Herein, we report a Pd(II)-catalyzed enantioselective C-H olefination toward the synthesis of P-stereogenic phosphinamides using cheap commercially available L-pGlu-OH as a chiral ligand. A broad range of P-stereogenic phosphinamides were gained in good yields with high enantioselectivities (33 examples, up to 77% yield, 99% ee) via desymmetrization and kinetic resolution.

Nickel-Catalyzed Asymmetric Synthesis of P-Stereogenic Phosphanyl Hydrazine Building Blocks

Catalytic asymmetric methods for the synthesis of synthetically versatile P-stereogenic building blocks offer an efficient and practical approach for the diversity-oriented preparation of P-chiral phosphorus compounds. Herein, we report the first nickel-catalyzed synthesis of P-stereogenic secondary aminophosphine-boranes by the asymmetric addition of primary phosphines to azo compounds. We further demonstrate that the P-H and P-N bonds on these phosphanyl hydrazine building blocks can be reacted sequentially and stereospecifically to access various P-stereogenic compounds with structural diversity.

Copper-Phosphido Catalysis: Enantioselective Addition of Phosphines to Cyclopropenes

We describe a copper catalyst that promotes the addition of phosphines to cyclopropenes at ambient temperature. A range of cyclopropylphosphines bearing different steric and electronic properties can now be accessed in high yields and enantioselectivities. Enrichment of phosphorus stereocenters is also demonstrated via a Dynamic Kinetic Asymmetric Transformation (DyKAT) process. A combined experimental and theoretical mechanistic study supports an elementary step featuring insertion of a CuI -phosphido into a carbon-carbon double bond. Density functional theory calculations reveal migratory insertion as the rate- and stereo-determining step, followed by a syn-protodemetalation.

Palladium-Catalyzed Regio- and Enantioselective Hydrophosphination of gem-Difluoroallenes

Chiral allylic phosphines and gem-difluoroalkenes are both important structural motifs in various bioactive molecules, chiral ligands, and natural products. These two motifs are now integrated, and we herein report a straightforward and atom-economical enantioselective hydrophosphination of gem-difluoroallenes using disubstituted phosphines. A wide array of enantioenriched fluorinated allylic phosphines has been accessed with excellent regio- and enantioselectivity and high efficiency. Synthetic and catalytic applications of phosphine products have been demonstrated.

Copper-Catalyzed Dynamic Kinetic Resolution of Secondary Phosphine Oxides

Copper-catalyzed dynamic kinetic resolution of secondary phosphine oxides has been successfully developed, providing a general method for the gram-scale enantioselective synthesis of P-stereogenic cyclic phosphine oxides with high yields and high enantioselectivities. The products could be easily reduced to the corresponding useful P(III)-stereogenic cyclic phosphines. A mechanism of the dynamic kinetic resolution involving the unusual rapid racemization of SPOs has been proposed.

Copper-Catalyzed Asymmetric Boroprotonation of Phosphinylallenes

Synthesis of chiral phosphorus compounds from readily available substrates by a facile method is an attractive strategy. In this study, an efficient route for copper-catalyzed asymmetric boroprotonation of phosphinylallenes with bis(pinacolato)diboron with high regioselectivity was developed, affording chiral allylphosphine oxides in high yields with high enantioselectivities of up to 98% ee. The synthetic utility was further demonstrated by the facile transformation of the chiral allylphosphine oxides to several stereospecific products.

Access to Enantiomerically Pure P-Stereogenic Primary Aminophosphine Sulfides under Reductive Conditions

Stereochemically pure phosphines with phosphorus-heteroatom bonds and P-centered chirality are a promising class of functional building blocks for the design of chiral ligands and organocatalysts. A route to enantiomerically pure primary aminophosphine sulfides was opened through stereospecific reductive C-N bond cleavage of phosphorus(V) precursors by lithium in liquid ammonia. The chemoselectivity of the reaction as a function of reaction time, substrate pattern, and chiral auxiliary was investigated. In the presence of exclusively aliphatic groups bound to the phosphorus atom, all competing reductive side reactions are totally prevented. The absolute configurations of all P-stereogenic compounds were determined by single-crystal X-ray diffraction analysis. Their use as synthetic building blocks was demonstrated. The lithium salt of (R)-BINOL-dithiophosphoric acid proved to be a useful stereochemical probe to determine the enantiomeric purity. Insights into the coordination mode of the lithium-based chiral complex formed in solution was provided by NMR spectroscopy and DFT calculations.

Secondary Phosphine Sulfide-Enabled Iridium-Catalyzed Asymmetric Allylic Substitution

An iridium-catalyzed asymmetric synthesis of branched allylic phosphine compounds under mild conditions is reported. Products bearing various functional groups can be synthesized with excellent stereoselectivity (up to 99.9 % ee) and regioselectivity. The employment of phosphine sulfides with relatively low deactivation capacity against metal catalysts is crucial for the success of this reaction.

Catalytic Asymmetric Axially Chiral Allenyl C-P Bond Formation

Chiral organophosphorous compounds are very important in catalysis, organic syntheses, and medicinal chemistry. However, catalytic enantioselective protocols for the axially chiral allenyl phosphorus compounds have never been reported. Herein, a palladium-catalyzed enantioselective carbon-phosphorus bond formation reaction affording axially chiral allenyl phosphonates has been developed. The reaction enjoys high yields and ees accommodating a wide range of functional groups. Mechanistic studies have unveiled an overwhelming kinetic resolution process.

Direct visible-light-induced synthesis of P-stereogenic phosphine oxides under air conditions

Over the past two decades, visible-light-induced transformations have been regarded as being among the most environmentally benign and powerful strategies for constructing complex molecules and diverse synthetic building blocks in organic synthesis. However, the development of efficient photochemical processes for assembling enantiomerically pure molecules remains a significant challenge. Herein, we describe a simple and efficient visible-light-induced C-P bond forming reaction for the synthesis of P-chiral heteroaryl phosphine oxides in moderate to high yields with excellent ee values (97-99% ee). Even in the absence of transition metal or photoredox catalysts, a variety of P-chiral heteroaryl phosphine oxides, including chiral diphosphine oxide 41, have been directly obtained under air conditions. Density functional theory (DFT) calculations have shown that the reaction involves intersystem crossing and single electron transfer to give a diradical intermediate under visible light irradiation.

Ni-catalyzed asymmetric hydrophosphinylation of conjugated enynes and mechanistic studies

The catalytic asymmetric synthesis of P-stereogenic phosphines is an efficient strategy to access structurally diverse chiral phosphines that could serve as organocatalysts and ligands to transition metals and motifs of antiviral drugs. Herein, we describe a Ni catalyzed highly regio and enantioselective hydrophosphinylation reaction of secondary phosphine oxides and enynes. This method afforded a plethora of alkenyl phosphine oxides which could serve as valuable precursors to bidentate ligands. A new type of mechanism was discovered by combined kinetic studies and density functional theory (DFT) calculations, which was opposed to the widely accepted Chalk-Harrod type mechanism. Notably, the alkene moiety which could serve as a directing group by coordinating with the Ni catalyst in the transition state, plays a vital role in determining the reactivity, regio and enantioselectivity.

Copper(I)-Catalyzed Asymmetric Synthesis of P-Chiral Aminophosphinites

Herein, a copper(I)-catalyzed reaction of diarylphosphines and O-benzoyl hydroxylamines is developed. In the cases of symmetrical diarylphosphines, a series of aminophosphinites is prepared in high yields. In the cases of unsymmetrical diarylphosphines, an array of P-chiral aminophosphinites is synthesized in high yields with high enantioselectivity by using a copper(I)-(R,R_P )-Ph-FOXAP complex as a chiral catalyst. Based on several control experiments and ³¹ P NMR studies, a two-electron redox mechanism involving the dynamic kinetic asymmetric transformation of unsymmetrical diarylphosphines is proposed for the copper(I)-catalyzed asymmetric reaction. Finally, one representative P-chiral phosphoric amide generated through the oxidation with H₂ O₂ is transformed to a chiral diarylphosphinate in high yield with retained enantioselectivity, which allows further transformations towards various P-chiral tertiary phosphines.

Rh-Catalyzed Regio- and Enantioselective Allylic Phosphinylation

Transition-metal-catalyzed branched and enantioselective allylic substitution of monosubstituted precursors with carbon, nitrogen, oxygen, sulfur, and fluoride nucleophiles has been well-established. However, such a selective carbon-phosphorus bond formation has not been realized probably due to the catalyst deactivation by the strong coordinating nature of phosphinylating reagents. Herein, we report a Rh-catalyzed highly regio- and enantioselective synthesis of allylic phosphine oxides in the presence of a chiral bisoxazoline-phosphine ligand. The application of α-hydroxylalkylphosphine oxides to keep the low concentration of the secondary phosphine oxides is essential for the high yields. The addition of diphenyl phosphoric acid was found to not only activate allylic alcohols but also accelerate the carbon-phosphorus bond formation.

Manganese(i)-catalyzed access to 1,2-bisphosphine ligands

Chiral bisphosphine ligands are of key importance in transition-metal-catalyzed asymmetric synthesis of optically active products. However, the transition metals typically used are scarce and expensive noble metals, while the synthetic routes to access chiral phosphine ligands are cumbersome and lengthy. To make homogeneous catalysis more sustainable, progress must be made on both fronts. Herein, we present the first catalytic asymmetric hydrophosphination of α,β-unsaturated phosphine oxides in the presence of a chiral complex of earth-abundant manganese(i). This catalytic system offers a short two-step, one-pot synthetic sequence to easily accessible and structurally tunable chiral 1,2-bisphosphines in high yields and enantiomeric excess. The resulting bidentate phosphine ligands were successfully used in asymmetric catalysis as part of earth-abundant metal based organometallic catalysts.

Chiral bisphosphine ligands are of key importance in transition-metal-catalyzed asymmetric synthesis of optically active products. Mn(i)-catalyzed hydrophosphination offers a two-step, one-pot synthetic sequence to access chiral 1,2-bisphosphines.

Nickel/Brønsted acid dual-catalyzed regio- and enantioselective hydrophosphinylation of 1,3-dienes: access to chiral allylic phosphine oxides

While chiral allylic organophosphorus compounds are widely utilized in asymmetric catalysis and for accessing bioactive molecules, their synthetic methods are still very limited. We report the development of asymmetric nickel/Brønsted acid dual-catalyzed hydrophosphinylation of 1,3-dienes with phosphine oxides. This reaction is characterized by an inexpensive chiral catalyst, broad substrate scope, and high regio- and enantioselectivity. This study allows the construction of chiral allylic phosphine oxides in a highly economic and efficient manner. Preliminary mechanistic investigations suggest that the 1,3-diene insertion into the chiral Ni-H species is a highly regioselective process and the formation of the chiral C-P bond is an irreversible step.

Ni-Catalyzed Enantioselective Benzylation of Secondary Phosphine Oxide

A nickel-catalyzed benzylic substitution of secondary phosphine oxide was described, affording the dialkylated P-stereogenic tertiary phosphine oxides with high to excellent enantioselectivities. The reaction was performed under mild conditions with commercially available benzyl chlorides and bench stable secondary phosphine oxides, exhibiting broad functional group tolerance. It represented a practical example for the preparation of P-stereogenic phosphine compounds.

Cobalt-Catalysed Asymmetric Addition and Alkylation of Secondary Phosphine Oxides for the Synthesis of P-Stereogenic Compounds

The catalytic asymmetric synthesis of P-chiral phosphorus compounds is an important way to construct P-chiral ligands. Herein, we report a new strategy that adopts the pyridinyl moiety as the coordinating group in the cobalt-catalysed asymmetric nucleophilic addition/alkylation of secondary phosphine oxides. A series of tertiary phosphine oxides were generated with up to 99 % yield and 99.5 % ee, and with broad functional-group tolerance. Mechanistic studies reveal that (R)-secondary phosphine oxides preferentially interact with the cobalt catalysts to produce P-stereogenic compounds.

Ni-Catalyzed Enantioselective Allylic Alkylation of H-Phosphinates

The asymmetric synthesis of P-stereogenic phosphinates through allylic alkylation of H-phosphinates has been developed. With H-phosphinates and allylic acetates as the starting materials, a variety of allylic P-chiral phosphinates were accessed in high enantioselectivities of up to 92% ee and generally high yields. In addition, a further study demonstrated the applicability of this protocol, including the scale-up synthesis and facile transformation of chiral products from phosphinates to phosphine oxides with organolithium reagents under mild reaction conditions.

Nickel-Catalyzed Asymmetric Synthesis of P-Stereogenic Vinyl Phosphines

Addition reaction to alkynes is an efficient strategy for constructing valuable alkenyl compounds. However, the elusive regioselectivity has been a persistent challenge. In the context of hydrophosphination reaction which could afford valuable P-stereogenic phosphines, the control of enantioselectivity as well as regioselectivity were especially tricky. Here, we highlighted our recent work on the nickel-catalyzed regio- and enantioselective hydrophosphination of unactivated alkynes with in situ generated secondary phosphines.

1 Introduction

2 Hydrophosphination of Alkynes

3 Derivatization Reactions

4 Mechanism Research

5 Summary and Outlook

Palladium/Xiao-Phos-Catalyzed Kinetic Resolution of sec-Phosphine Oxides by P-Benzylation

P-stereogenic tert- and sec-phosphines have wide applications in asymmetric catalysis, materials, and pharmaceutical chemistry, however, their practical synthesis still constitutes a significant challenge. Herein, a successful kinetic resolution of rac-secondary phosphine oxides via the enantioselective P-benzylation process catalyzed by the palladium/Xiao-Phos was designed. Both tert- and sec-phosphine oxides were delivered in good yield and excellent enantiopurity (selectivity factor up to 226.1). The appealing synthetic utilities are further demonstrated by the facile preparation of several valuable P-chiral compounds, precursors of bidentate ligands, as well as transition metal complexes.

Enantioselective Palladium-Catalyzed Hydrophosphinylation of Allenes with Phosphine Oxides: Access to Chiral Allylic Phosphine Oxides

A Pd-catalyzed hydrophosphinylation of alkyl and aryl-oxyallenes with phosphine oxides has been developed for the efficient and rapid construction of a family of chiral allylic phosphine oxides with a diverse range of functional groups. This methodology was further applied in the facile construction of chiral 2H-chromene and later stage functionalization of cholesterol.